We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature T C 3 K and a small ordered moment m 0 0:03 B . Superconductivity is observed with a resistive transition temperature T s 0:8 K for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of T s , and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations. DOI: 10.1103/PhysRevLett.99.067006 PACS numbers: 74.70.Tx, 74.20.Mn, 75.30.Kz In the standard theory for superconductivity (SC) due to Bardeen, Schrieffer, and Cooper ferromagnetic (FM) order impedes the pairing of electrons in singlet states [1]. It has been argued, however, that on the border line of ferromagnetism, critical magnetic fluctuations could mediate SC by pairing the electrons in triplet states [2]. The discovery several years ago of SC in the metallic ferromagnets UGe 2 (at high pressure) [3], URhGe [4], and possibly UIr (at high pressure) [5], has put this idea on firm footing. However, later work provided evidence for a more intricate scenario in which SC in UGe 2 and URhGe is driven by a magnetic transition between two polarized phases [6 -8] rather than by critical fluctuations associated with the zero temperature transition from a paramagnetic to a FM phase. Here we report a novel ambient-pressure FM superconductor UCoGe. Since SC occurs right on the border line of FM order, UCoGe may present the first example of SC stimulated by critical fluctuations associated with a FM quantum critical point (QCP).UCoGe belongs to the family of intermetallic UTX compounds, with T a transition metal and X is Si or Ge, that was first manufactured by Troć and Tran [9]. UCoGe crystallizes in the orthorhombic TiNiSi structure (space group P nma ) [10,11], just like URhGe. From magnetization, resistivity (T 4:2 K) [9,10] and specific-heat measurements (T 1:2 K) [12] it was concluded that UCoGe has a paramagnetic ground state. This provided the motivation to alloy URhGe (Curie temperature T C 9:5 K) with Co in a search for a FM QCP in the series URh 1ÿx Co x Ge (x 0:9) [13]. Magnetization data showed that T C upon doping first increases, has a broad maximum near x 0:6 (T max C 20 K) and then rapidly drops to 8 K for x 0:9 [13]. This hinted at a FM QCP for x & 1:0. In this Letter we show that the end (x 1:0) compound UCoGe is in fact a weak itinerant ferromagnet. Moreover, metallic ferromagnetism coexists with SC below 0.8 K at ambient pressure.Polycrystalline UCoGe samples were prepared with nominal compositions U 1:02 CoGe (sample 2) and U 1:02 Co 1:02 Ge (sample 3) by arc melting the constituents (natural U 99.9%, Co 99.9%, and Ge 99.999%) under a high-purity argon atmosphere in a water-cooled copper crucible. The as-cast samples were annealed for 10 days at 850 C. Sampl...

We report upper critical field B(c2)(T) measurements on a single-crystalline sample of the ferromagnetic superconductor UCoGe. B(c2)(0) obtained for fields applied along the orthorhombic axes exceeds the Pauli limit for B parallela,b and shows a strong anisotropy B(c2)(a) approximately B(c2)(b)>>B(c2)(c). This provides evidence for an equal-spin pairing state and a superconducting gap function of axial symmetry with point nodes along the c axis, which is also the direction of the uniaxial ferromagnetic moment m(0)=0.07micro(B). An unusual curvature or kink is observed in the temperature variation of B(c2) which possibly indicates UCoGe is a two-band ferromagnetic superconductor.

We report zero-field muon-spin rotation and relaxation measurements on the superconducting ferromagnet UCoGe. Weak itinerant ferromagnetic order is detected by a spontaneous muon-spin precession frequency below the Curie temperature TC=3 K. The micro+ precession frequency persists below the bulk superconducting transition temperature Tsc=0.5 K, where it measures a local magnetic field Bloc=0.015 T. The amplitude of the microSR signal provides unambiguous proof for ferromagnetism present in the whole sample volume. We conclude ferromagnetism coexists with superconductivity on the microscopic scale.

We investigate the effect of substituting Si for Ge in the ferromagnetic superconductor UCoGe. Dc-magnetization, ac-susceptibility and electrical resistivity measurements on polycrystalline UCoGe1−xSix samples show that ferromagnetic order and superconductivity are progressively depressed with increasing Si content and simultaneously vanish at a critical concentration xcr ≃ 0.12. The non-Fermi liquid temperature variation in the electrical resistivity near xcr and the smooth depression of the ordered moment point to a continuous ferromagnetic quantum phase transition. Superconductivity is confined to the ferromagnetic phase, which provides further evidence for magnetically mediated superconductivity.

We have investigated the evolution of ferromagnetic order in the correlated metal series URh 1-x Ru x Ge. Magnetization, transport and specific heat measurements provide convincing evidence for a ferromagnetic quantum critical point at the critical concentration x c = 0.38. Here we report ac-susceptibility and magnetization measurements on selected samples with Ru doping concentrations near the critical point.Keywords: URhGe; ferromagnetism; chemical substitution; quantum phase transition The intermetallic compound URhGe attracts much attention because itinerant ferromagnetism (T C = 9.5 K) and unconventional superconductivity (T s = 0.25 K) coexist [1]. Moreover, re-entrant superconductivity is observed for a magnetic field B ≈ 12 T applied along the orthorhombic b-axis [1]. A natural explanation for the occurrence of these unusual superconducting states is that critical magnetic fluctuations stimulate the formation of spintriplet Cooper pairs. Therefore it is of considerable interest to explore the magnetic properties of URhGe, especially as far as the ferromagnetic (FM) instability is concerned.Recently, we have investigated the evolution of FM order in URhGe doped with Ru, Co and Si [2]. Magnetization and transport experiments on a series of polycrystalline URh 1-x Ru x Ge samples down to T = 2 K showed that FM order, after an initial weak increase, is suppressed in a linear fashion and disappears at a critical concentration x c = 0.38 (see Fig.1). In fact these experiments hinted at the existence of a FM quantum critical point (QCP) in the U(Rh,Ru)Ge system. This was corroborated by subsequent low temperature (T ≥ 0.3 K) specific-heat and resistivity measurements [3]. At x c the phononcorrected specific heat varies as c ~ Tln(T/T 0 ) over a wide T range and γ(x) (i.e. (c/T)| 0.5K ) goes through a pronounced maximum. The critical behaviour is also tracked by the exponent of the resistivity ρ ~ T n , which attains a minimum value n = 1.2 at x c . Taking into account that the magnetization data show a smooth suppression of the ordered moment m 0 (x), these results provide convincing evidence for a continuous FM quantum phase transition in the U(Rh,Ru)Ge system [3].In this work we further investigate the phase diagram in the vicinity of the critical point. We report low temperature (T ≥ 0.23 K) ac-susceptibility, χ ac , measurements in a field of 10 -5 T. The data were taken on arc-melted polycrystalline samples annealed at 875 o C for 10 days. In Fig.2 we show the results for x = 0.35 and 0.375. The maxima in χ ac signal FM